Dry out (5)
'dry out' is the definition.
(dehydrating is a kind of drying out)
This is the entire clue.
(Other definitions for dehydrate that I've seen before include "Remove moisture content from, dry out" , "Preserve by removing all liquid from" , "Drain" , "Lose a lot of water" , "Remove moisture from, make dry" .)
'dry out' is the definition.
(parching is a kind of drying out)
This is all the clue.
(Other definitions for parch that I've seen before include "Cut off water supply" , "Make dry through heat" , "Scorch" , "Make hot and dry" , "Get overheated" .)